I'm working on a few small games, and I was wondering if I should use American or British English for my user interface? I'm not targetting any specific country, and this will probably be the only localization.
As much as it pains me to say it (as a Brit), if you're not going to have any localisation go with American spellings. This will be the preferred spellings for the vast majority of users - either as native speakers (Americans far outnumber the British) or as second language speakers (though there are significant number of those who use British English spellings).
The only exception would be if you are going for a "quirky, English vibe" for your game.